Recently repaired the tank vent on my 57. As the gas cap is vented & the gasket isn't good, I decided to replace it with a non vented cap. However, am I labouring under a misconception? I thought that most modern cars had semi closed fuel systems & non vented gas caps. When I went to buy a cap from our local parts retailer, most of the caps they have on hand, are vented. What gives?   

Paul